1. Field of the Invention
The present invention relates to spindle motors for disk drives. More particularly, the present invention relates to a disk drive comprising a multi-phase spindle motor having a center tap connectable to a secondary supply voltage.
2. Description of the Prior Art
Disk drives typically employ a multi-phase spindle motor (e.g., a three phase spindle motor) for spinning a disk while a head writes data to and reads data from the disk. The head is connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) in order to actuate the head radially over the disk to access radially spaced, concentric tracks. The disk drive receives power from a host computer, including a 12V supply for powering the spindle motor and VCM, and a 5V supply for powering the integrated circuits that control the operation of the disk drive. The amount of current the disk drive can draw from either supply is limited so as not to interfere with the proper operation of the host computer.
Two important performance metrics for a disk drive are the amount of time required for the spindle motor to spin up the disk to its operating RPM, and the seek time required for the VCM to position the head over a target track. A short spin up period is desirable not only to enhance the user's experience, but also to satisfy the time-out restriction imposed by the host computer. That is, the host computer will assume an error occurred if the disk drive does not return with a “ready” status within a prescribed time interval during a power on cycle. A short seek time is also desirable since the mechanical latency of the VCM dominates the overall access time. Both the spin up and seek times are limited by the amount of current that can be drawn from the 12V supply as well as the efficiency (cost) of the spindle motor and VCM. In addition, the inertia of the disk as determined from its physical characteristics as well as the operating RPM limit the spin up time. The spin up time increases as the form factor and operating RPM increase.
For higher RPM (e.g., 10,000 RPM) prior art disk drives typically employ a smaller form factor disk (e.g., 3 inch rather than 3.5 inch diameter disk) to achieve performance with respect to the spindle motor without exceeding the current limitations of the 12V supply. However, a smaller form factor requires more disks to achieve higher capacity which increases the cost and height of the disk drive.
U.S. Pat. No. 5,532,563 discloses a disk drive having an auxiliary power source used to augment the 12V supply to decrease the spin up time. However, the auxiliary power source increases the complexity and expense of the disk drive, and the auxiliary power source is recharged from the 12V supply which means during a power on sequence there is a charging delay before the spindle motor can begin the spin up process.
U.S. Pat. No. 5,381,279 discloses a disk drive wherein the current applied to the spindle motor is reduced during the acceleration and deceleration intervals of a seek with a corresponding increase in current to the VCM. During a coast period of the seek (between the acceleration and deceleration intervals) the current applied to the spindle motor is increased with a corresponding decrease in the current applied to the VCM. This enables more aggressive seek profiles while maintaining the spindle speed at its operating RPM without exceeding the current limit of the 12V supply. Although this technique reduces the seek and access times, it does not improve performance with respect to the spin up time. Further, abruptly changing the current supplied to the spindle motor creates undesirable acoustic vibrations audible to the end user.
There is, therefore, a need to cost effectively decrease the spin up and seek times in a disk drive without exceeding the current limitations of the host computer.